With the advancement of nanoscale fabrication, label-free sensors such as photonic crystal, whispering gallery mode (“WGM”) and surface plasmon resonance (“SPR”) based devices are increasingly being used for detecting DNA bases, recognizing antigen-antibody, probing protein interaction, immunoassays and identifying pathogens. Most of the above sensors work on the principle of shift in the resonance wavelength after conjugation of biomolecules to the surface of the sensor. However, the resonance peak wavelength shift (“PWS”) for most of the sensors known in the art is on the order of only a few nanometers. SPR sensors do have a higher sensitivity of 2×106 nm per refractive-index unit (“RIU”) for bulk refractive index, yet the absolute change in wavelength is extremely minimal for known SPR sensors in a Kretschmann configuration. As a result, the identification of the unknown target analyte (or concentration of a known target analyte) requires specialized instrument such as high precision spectrometer, multiplexer for sweeping the laser wavelengths, WGM trap, complex optical system to generate SPR (for example, prism, accurate angle tuning for the optical beam) etc.
Localized surface plasmon resonance (“LSPR”) sensors, based on colloidal plasmonic particles (e.g., silver or gold), overcome some of the above limitations. However, the sensitivity of LSPR based sensors are at least an order of magnitude less compared to the gold standard prism coupled SPR sensors (Kretschmann configuration). In the LSPR configuration, the scattering spectra of the plasmonic particles shift to a different wavelength (usually to a longer wavelength) after conjugation to specific analytes, but the colloid particle sizes and positions are random and difficult to control over a large area. Thus, in the same sample area, different particles give rise to different scattering spectra such that the shifts in the wavelength are different for the same analyte. In addition, a high precision spectrometer is required to record the spectra and then complex image analysis is required to extract usable data.
Another known method to obtain high sensitivity plasmonic resonance is to make sub-wavelength holes in optically thick metal surfaces, commonly referred to as extraordinary optical transmission (“EOT”) substrates. The fabrication of such substrates relies on expensive, time consuming and low throughput electron beam lithography and focused ion beam milling. Also, due to presence of single layer of metal surface, usually the nonradiative Drude (ohmic) damping losses are high, leading to damping of resonance with analytes on the top of the metal surface. In addition, EOT substrates generally show multiple transmission peaks in the visible range, making it difficult for true colorimetric sensing modalities. Quasi 3-D plasmonic crystals offer a way to increase the sensitivity by employing multiple layers of metal surfaces on nanohole surfaces. However, the resonance of such devices has been mostly demonstrated in near infrared and far infrared wavelength. Also, after adsorption of analyte, minimal to no resonance shift is observed. Most of the analyte detection or quantification is accomplished through the change in the infrared transmission intensities.